Heterodontosaurus was a small, fleetfooted ornithischian that reached a maximum size of about 3 feet. It had a long, narrow pelvis and a pubis which resembled those possessed by more advanced ornithischians. More unusual was that the hand of Heterodontosaurus had five fingers, two of which seem to be opposable. This configuration allowed Heterodontosaurus to grasp and manipulate food. The bone in the foot and ankle were fused in a manner reminiscent of those in birds.
Another interesting feature is the specialization of teeth which gave rise to the animal's name. Most dinosaurs (and indeed most reptiles) have a single type of tooth in their jaws, while Heterodontosaurus had three. At the front of the jaw beside the beak were small teeth likely used for chopping off leaves and stems. Next in the jaw was a large pair of tusks whose purpose is unknown, but it is speculated that they were used as sexual displays (where the tusks could have been used as weapons by rival males in disputes over mates and territories) or to break open prehistoric termite mounds. The final type of teeth were tall and squared off. This type of teeth was well adapted for chewing. Fleshy cheeks helped keep the food in the mouth while chewing occurred. Chewing is relatively common in dinosaurs, but uncommon for other groups of reptiles.This bizarre suite of teeth has led to debate over what heterodontosaurs ate. Some scientists think heterodontosaurs were omnivores who used their differently-shaped teeth to eat both plants and small animals.